Google Photos was launched 2 months ago and there are many missing features. For examples, some powerful editing features from Google+ Photos for desktop are not yet available in Google Photos, global Auto Enhance is missing and you can't cast photos, organize albums, edit location and date taken or tag people in Google Photos.
Since both services are 2 interfaces that show almost the same photos and videos, you can find most of your files in Google Photos. Apparently, photos shared in Blogger and Hangouts are only available in Picasa Web Albums, for now. If you don't like Google's new service, there's always Google Takeout, which lets you export all your photos and videos.